None.
The present invention relates to microfabricated structures to interact with electromagnetic waves and, more particularly to addressable, reusable visual displays. Still more particularly, an embodiment of the invention relates to preformed microstructured substrates containing assisting optical elements to enhance the visual effect of visual displays, such as gyricon displays using rotatable particles (e.g., rotary balls).
For purpose of illustration, the present application uses structures of gyricon displays to demonstrate the concepts and the benefits of the inventive structure.
A gyricon display, also called a twisting-particle display, rotary ball display, particle display, dipolar particle light valve, etc., is a type of addressable visual displays. A gyricon display offers a technology for making a form of electric paper and other reflective displays. Briefly, a gyricon display is an addressable display made up of a multiplicity of optically anisotropic particles, with each particle being selectively rotatable to present a desired face to an observer. The rotary particle can be of various shapes, such as spherical or cylindrical. For convenience, balls, rather than cylinders, are used in this description for illustrations.
Addressable visual displays typically have multiple display units such as pixels or subpixels. A separate assisting optical element is sometimes used in connection with each display to enhance or create certain visual effect. U.S. Pat. No. 5,777,782 to Sheridon, for example, discloses a gyricon or rotating-particle display having an auxiliary optical structure which is a pre-formed array of lenses indexed to gyricon particles. Although the Sheridon patent relates to gyricon displays only, in principle the use of an auxiliary optical structure is not limited to the gyricon displays. A properly designed auxiliary optical structure may be used to enhance or create certain visual effects in other types of visual displays containing multiple display units, such as displays using electronic ink based on the electrophoretic principle made by E Ink Corp. For purpose of illustration, however, the present application uses structures of gyricon displays to demonstrate the concepts and the benefits of the inventive structure.
A gyricon display, also called a twisting-particle display, rotary ball display, particle display, dipolar particle light valve, etc., offers a technology for making a form of electric paper and other reflective displays. Briefly, a gyricon display is an addressable display made up of a multiplicity of optically anisotropic particles, with each particle being selectively rotatable to present a desired face to an observer. The rotary particle can be of various shapes, such as spherical or cylindrical. For convenience, balls, rather than cylinders, are used in this description for illustrations.
Like ordinary paper, electric paper preferably can be written on and erased, can be read in ambient light, and can retain imposed information in the absence of an electric field or other external retaining force. Also like ordinary paper, electric paper preferably can be made in the form of a lightweight, flexible, durable sheet that can be folded or rolled into tubular form about any axis and can be conveniently placed into a shirt or coat pocket and then later retrieved, restraightened, and read substantially without loss of information. Yet unlike ordinary paper, electric paper preferably can be used to display full-motion and changing images as well as still images and text. Thus, it is particularly useful for bistable displays where real-time imagery is not essential, but also adaptable for use in real-time imaging such as a computer display screen or a television.
A gyricon display, also called a twisting-particle display, rotary ball display, particle display, dipolar particle light valve, etc., offers a technology for making a form of electric paper and other reflective displays. Briefly, a gyricon display is an addressable display made up of a multiplicity of optically anisotropic particles, with each particle being selectively rotatable to present a desired face to an observer. The rotary particle can be of various shapes, such as spherical or cylindrical. For convenience, balls, rather than cylinders, are used in this description for illustrations.
In the prior art, the black-and-white balls (particles) are embedded in a sheet of optically transparent material, such as an elastomer sheet. The elastomer sheet is then cured. After curing, the elastomer sheet is placed in a plasticizer liquid, such as a dielectric fluid. The dielectric plasticizer swells the elastomer sheet containing the particles creating cavities larger than the particles around the particles. The cavities are also filled with the absorbed dielectric fluid. The fluid-filled cavities accommodate the particles, one particle per cavity, so as to prevent the particles from migrating within the sheet.
Besides being optically anisotropic, the particles are electrically dipolar in the presence of the fluid. This may be accomplished by simply including in one or both hemispheres materials that impart an electrical anisotropy, or by coating one or both sides of hemispheres with materials that impart electrical anisotropy. The above charge activation agents may impart an electrical anisotropy and an optical anisotropy at the same time. For example, when each hemisphere of a gyricon particle is coated with a material of a distinct electrical characteristic (e.g., Zeta potential with respect to a dielectric fluid) corresponding to a distinct optical characteristic the particles will have an electrical anisotropy in addition to their optical anisotropy when dispersed in a dielectric liquid. It is so because when dispersed in a dielectric liquid the particles acquire an electric charge related to the Zeta potential of their surface coating.
An optically anisotropic particle can be selectively rotated within its respective fluid-filled cavity, for example by application of an electric field, so as to present either its black or white hemisphere to an observer viewing the surface of the sheet. Under the action of an addressing electric field, such as provided by a conventional matrix addressing scheme, selected ones of the optically and electrically anisotropic particles are made to rotate or otherwise shift their orientation within their cavities to provide a display by the selective absorption and reflection of ambient light. Since the particles need only rotate, not translate, to provide an image, much faster imaging response is achieved than with the display of U.S. Pat. No. 3,612,758.
When the electric field is applied to the sheet, the adhesion of each particle to the cavity is overcome and the particles are rotated to point either their black or white hemispheres towards the transparent surface. Even after the electric field is removed, the structures (particles in specific orientations) will stay in position and thus create a bistable display until the particles are dislodged by another electric field. An image is formed by the pattern collectively created by each individual black and white hemisphere. Thus, by the application of an electric field addressable in two dimensions (as by a matrix addressing scheme), the black and white sides of the particles can be caused to appear as the image elements (e.g., pixels or subpixels) of a displayed image. These bistable displays are particularly useful for remotely addressable displays that require little power to switch and no power to maintain display image for a long period of time (e.g., months).
Gyricon display technology is described further in U.S. Pat. No. 4,126,854 (Sheridon, xe2x80x9cTwisting Ball Panel Displayxe2x80x9d) and U.S. Pat. No. 5,389,945 (Sheridon, xe2x80x9cWriting System Including Paper-Like Digitally Addressed Media and Addressing Device Thereforxe2x80x9d). Further advances in black and white gyricon displays have been described in U.S. Pat. No. 6,055,091 (Sheridon, xe2x80x9cTwisting-Cylinder Displayxe2x80x9d). The above-identified patents are all hereby incorporated by reference. The Sheridon patent disclosed a gyricon display which uses substantially cylindrical bichromal particles rotatably disposed in a substrate. The twisting cylinder display has certain advantages over the rotating ball gyricon because the elements can achieve a much higher packing density. The higher packing density leads to improvements in the brightness of the twisting cylinder display as compared to the rotating ball gyricon.
Gyricon displays are not limited to black and white images, as gyricon and other display mediums are known in the art to have incorporated color. Gyricons incorporating color have been described in U.S. Pat. No. 5,760,761 titled xe2x80x9cHighlight Color Twisting Ball Displayxe2x80x9d, U.S. Pat. No. 5,751,268 titled xe2x80x9cPseudo-Four Color Twisting Ball Displayxe2x80x9d, U.S. patent application Ser. No. 08/572,820 titled xe2x80x9cAdditive Color Transmissive Twisting Ball Displayxe2x80x9d, U.S. patent application Ser. No. 08/572,780 titled xe2x80x9cSubtractive Color Twisting Ball Displayxe2x80x9d, U.S. Pat. No. 5,737,115 titled xe2x80x9cAdditive Color Tristate Light Valve Twisting Ball Displayxe2x80x9d, U.S. Pat. No. 6,128,124 titled xe2x80x9cAdditive Color Electric Paper Without Registration or Alignment of Individual Elementsxe2x80x9d and European Patent No. EP0902410 titled xe2x80x9cMethods for Making Spinnable Ball, Display Medium and Display Devicexe2x80x9d. The above-identified patents are all hereby incorporated by reference.
The above prior art all involve a process which is to randomly pack the bichromal particles in an elastomeric matrix, cure the elastomer, and subsequently swell the elastomer in the dielectric oil. The process is laborious and time-consuming, consisting of mixing of the particles into the elastomer, coating the slurry into a sheet format, curing, and subsequently swelling the sheet with the dielectric oil.
Furthermore, the display device of such an arrangement poses problems in connection with the selection of a usable dielectric liquid, stability upon changes in temperature, non-uniformity of dimensions of the cavities, and the like. The material considerations in the prior art are many, the primary issues being tuning the swelling of the elastomer by the dielectric oil without harming the dielectric oil compatibility with all the other elements of the display package.
Furthermore, the above approach resulted in less than satisfactory contrast of the display, associated with the relatively low reflectance of a gyricon display. It is commonly believed that the best way to improve the reflectance of a gyricon display is to make the display from a close packed arrangement of bichromal particles. The closer packed the arrangement of particles, the better the reflectance and the brighter the appearance of the display. To achieve a close packed arrangement, the cavities in which the particles rotate should be close to each other and each cavity should have little unfilled space when filled with a particle, ideally no more empty space than what is necessary to keep the particle therein rotatable. The prior art approaches, however, had difficulties to achieve a high density of particles, mainly due to the lack of controlling on the formation of individual cavities. The result is typically that cavities are either too large, or distributed too loosely in the elastomer with large distances and thick walls between the individual cavities, making it difficult to control the arrangement and packing density of the display particle members to a sufficiently high value to achieve a display of high quality, high resolution, and high contrast.
As a related problem, in a typical conventional gyricon display, bichromal particles are dispersed throughout the thickness of the substrate sheet, which is always thicker than two particle diameters and is usually many diameters thick. Generally, less than 20 percent of the upper surface area of the sheet is covered by the bichromal particles in the layer closest to the surface. Therefore, a display according to the above prior art has multiple layers of particles instead of a single layer, making the display thick and bulky, an undesirable feature especially for an electronic paper. In the prior art designs, the multiple layer configuration is on one hand necessary in order to increase the reflectance (the reflectance of multiple layers of loosely packed particles accumulatively approaches that of a closely packed single layer) and on the other hand difficult to avoid due to the characteristics of the prior art process of making a display.
To achieve higher packing density, the above method was modified in U.S. Pat. No. 4,438,160 to Ishikawa et al, which patent is incorporated by reference. In the Ishikawa patent, instead of using the swelling method to create cavities larger than the particles, the particles are coated with a layer of wax before being placed in the elastomer. The wax is later melted away, resulting in cavities that are larger than the particles. Presumably, because it is easier to control the thickness of the wax layer coated on the particles than to control the degree of swelling the elastomer, it is also easier to achieve higher density of particles by using the Ishikawa method. The actual improvement, however, is not significant enough to solve the problem. See U.S. Pat. No. 5,825,529 to Crowley, which patent is incorporated by reference.
To achieve still higher packing density, a gyricon display can be constructed without elastomer and without cavities. U.S. Pat. No. 5,825,529 to Crowley, for example, uses no elastomer substrate. In the display in the Crowley patent, the bichromal particles are placed directly in the dielectric fluid. The particles and the dielectric fluid are then sandwiched between two retaining members (e.g., between the addressing electrodes). There is no elastomer substrate. Electrodes serve both to address particles and to retain particles and fluid in place. Particles and fluid can be sealed in the display by seals at either end of the display. In addition, the spacing between electrodes is set to be as close to the diameter of particles as is possible consistent with proper particle rotation, resulting a monolayer display. The Crowley patent achieved a display with a closely packed monolayer having a light reflectance that surpasses that of the multi-layer displays in the prior art. The Crowley patent achieved a display with a closely packed monolayer having a light reflectance that surpasses that of the multi-layer displays in the prior art. The display in Crowley, however, achieves a higher packing density by sacrificing structural integrity. The Crowley display lacks internal support and has insufficient sealing. Particularly, the display will not work when placed vertically.
More fundamentally, even with the above improved methods making twisting particle displays the particles cannot be packed together to completely fill the area of the display because of the existence of interstices. Furthermore, regardless of which microstructure is used, and regardless of how the particles are packed, the particles often do not exactly rotate to the precise orientation to have only the side with the desired optical characteristics facing the viewer. Both partial filling and partial rotating contribute to decreased image contrast in the following manner: Gyricon displays use optically anisotropic particles that are selectively rotatable to communicate visual information. For example, in a display using bichromal spherical balls where each ball defines a display unit which conveys the characteristic color information of the spherical ball""s hemisphere which is selectively turned to face the viewer, the unit display area is typically the projection area or image size of the ball. Due to the unfilled spaces between the particles and also due to imperfect rotation which may show wrong color or portions of contrasting (hence cancelling) colors, each particle is surrounded by a peripheral area which does not carry any color information of the particle selectively rotated. Instead the peripheral area substantially reflects the optical characteristic of the substrate which is typically dark. This phenomenon causes decreased contrast. The same phenomenon exists in displays where each unit display is defined by multiple particles.
The present invention uses assisting optical elements to enhance or improve an optical effect of a microstructure, such as contrast of visual displays (e.g., a gyricon display). The assisting optical elements may be either reflective or refractive. To enhance contrast of a visual display, for example, an assisting optical element is placed over or around each display unit to form an enlarged image of at least a portion of the upper side of the particles in that display unit so that the effective unit display area is larger than the actual unit display area. The actual unit display area is defined by the physical sizes of the particles. For example, in a display of one particle per pixel, when the entire particle is visible to the viewer from above, the actual unit display area is the actual size of the particle.
Assisting optical elements of various designs maybe used to achieve the above purpose. A reflective corona shouldering a particle, for example, creates an appearance of the surface of the particle larger than the actual size of the surface through reflection of the light from surface, given that the reflective corona is larger than the particle. A reflective corona may simply be made of metalized reflective surfaces, or alternatively formed by using the principle of total internal reflection in which a total reflection is created at an interface of two different materials at certain incident angles of the light, even though the interface is not made of a material which is highly reflective in ordinary sense. Alternatively, optical lenses may be used to form enlarged images of the surface of each particle when viewed from above. In this case, the light from the surface of the particle is spread to the peripheral area through refraction instead of reflection.